AQA GCSE Physics 4.3 Particle model of matter: a complete overview of density, internal energy, latent heat and gas pressure

What topic 4.3 actually demands

The particle model is the idea that all matter is made of particles whose arrangement and motion explain its behaviour. AQA tests two linked skills: using the particle model to explain density, changes of state and gas pressure, and calculating with the density, latent heat and pressure-volume relationships.

This guide walks through all four dot points of the topic, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Density

Density is mass per unit volume, $\rho = \frac{m}{V}$. The particle model explains why solids are dense (particles packed closely in a regular pattern), liquids slightly less dense (particles close but mobile) and gases very low in density (particles far apart). The required practical measures mass with a balance and volume by regular dimensions or by displacement.

Internal energy and changes of state

Internal energy is the total kinetic and potential energy of the particles. Heating raises internal energy, which either increases the temperature (more kinetic energy) or causes a change of state (more potential energy at constant temperature). The changes of state, melting, freezing, boiling, evaporating and condensing, are physical changes: the same particles remain, mass is conserved, and they are reversible.

Specific heat capacity and specific latent heat

Specific heat capacity is the energy to raise the temperature of $1\,kg$ by $1\,^{\circ}C$, used in $\Delta E = mc\Delta\theta$. Specific latent heat is the energy to change the state of $1\,kg$ with no temperature change, used in $E = mL$. The latent heat of fusion is for melting and freezing; the latent heat of vaporisation, which is larger, is for boiling and condensing.

Particle motion in gases

Gas particles move in rapid, random directions and collide with the container walls; the total force from these collisions over the wall area is the pressure. Higher temperature means higher average kinetic energy, so faster particles and higher pressure. For a fixed mass at constant temperature, $pV = \text{constant}$, so compressing a gas raises its pressure.

How topic 4.3 is examined

A typical AQA profile for the Particle model of matter:

• Short answer. Defining density, internal energy and specific latent heat, and describing particle arrangements.
• Calculations. Density, $\Delta E = mc\Delta\theta$, $E = mL$, and $p_1V_1 = p_2V_2$, often in multi-step problems.
• Required practical. The density method and sources of error.
• Extended answers. Explaining changes of state with the particle model, why temperature is constant during a change of state, and how particle motion creates gas pressure.

Check your knowledge

A mix of recall and calculation questions covering topic 4.3. Attempt them under timed conditions, then check against the solutions.

1. State the density equation and the units of each quantity. (2 marks)
2. A block has a mass of $600\,g$ and a volume of $200\,cm^3$. Calculate its density. (2 marks)
3. Define the internal energy of a system. (2 marks)
4. Explain why the temperature stays constant while water boils. (2 marks)
5. State the difference between the specific latent heat of fusion and of vaporisation. (2 marks)
6. Calculate the energy to melt $0.4\,kg$ of ice ($L = 334000\,J/kg$). (2 marks)
7. Explain how gas particles create pressure on the walls of a container. (2 marks)
8. A gas at $150\,kPa$ occupies $0.4\,m^3$. It is compressed to $0.1\,m^3$ at constant temperature. Calculate the new pressure. (3 marks)